LO – To understand the changes in river process with distance from source - To understand Long and Cross Profiles of a river

Long Profile If you needed to describe the graded profile you could describe it as a smooth, concave profile in dynamic equilibrium

3. THE GRADED PROFILE

Long Profile

Cross Profile Due to the changes in energy, erosion and deposition, the cross profile of the river will change as you move from source to mouth.

Upper Course In the upper course, the valley and channel are narrow and deep as a result of the large amount of vertical erosion and little lateral erosion. The sides of a river’s valley in the upper course are very steep earning these valleys the nickname “V-Shaped Valley” since they look like a letter V. The river’s valley can be anything from a few meters to a few hundred metres in width depending on the lithology but the channel rarely more than 5m or 6m wide.

Middle Course In the middle course, the valley has increased in width due to the increase in lateral erosion but its depth hasn’t changed significantly because vertical erosion has slowed down. Similarly, the channel’s width has increased but it’s still roughly the same depth. The land to either side of the channel in the valley is now the river’s floodplain and the valley’s sides are much more gentle.

Lower Course In the lower course the valley is now very wide (often several kilometres) and the floodplain has increased greatly in size. The channel is a little wider but not much deeper.

Complete activity 1-4 on P24-25 (Ross & Digby)

The cross-section of a valley will change from the upper to lower course Upper – erosion is dominant and a V shape is formed by the powerful downward or vertical erosion. The valley sides are also subjected to weathering and slope movements. Lower – transport and deposition are the main processes and the valley widens out

Discharge, width, depth, velocity and load all increase downstream What does this model show? Discharge, width, depth, velocity and load all increase downstream Particle size, bed roughness and slope angle all decrease downstream.

Terminology /

1. CHANNEL EFFICIENCY Measure of the ability of a river channel to move water and sediment. The most efficient channels are generally semicircular in cross-section, and it is this shape that water engineers try to create when altering a river channel to reduce the risk of flooding.

2. THE HYDRAULIC RADIUS The hydraulic radius is a measure of a river's channel efficiency, and is used by water engineers to assess the likelihood of flooding. The hydraulic radius of a channel is defined as the ratio of its cross-sectional area to its wetted perimeter. The greater the hydraulic radius, the greater the efficiency of the channel and the less likely the river is to flood. The highest values occur when channels are deep, narrow, and semi-circular in shape.

Hydraulic Radius This is important as it tells us how efficient the river is. It depends on how much the water is being slowed down by friction from the banks and bed. See how the banks/bed slow the water by friction in a random cross section. The air also slows water but to a much lesser degree.

For each channel…. Work out the wetted perimeter Work out the cross sectional area Work out the Hydraulic Radius Which river is more efficient? (The one with the greater HR)

Contrast the channel shapes and comment on the relative efficiency of the two river cross sections ( 7 marks)

Read pages 9 – 11 Smith and Knill. Complete activities 8 to 10 Read pages 9 – 11 Smith and Knill. Complete activities 8 to 10. Add any key terms to your glossary.

Recap - Hjulstrom Write on your white boards whether each statement is true or false

A silt particle with a diameter of 0 A silt particle with a diameter of 0.01mm requires an erosion velocity of 20cm/sec False (Approx 60cm/sec would be needed to erode this particle)

Sand particles range in size from just over 0.0004mm to about 0.07mm False (0.1mm to 1mm)

A particle with a diameter of 0 A particle with a diameter of 0.1mm will not be deposited until the velocity falls to about 1cm/sec True

For a particle with a diameter of 10mm, a velocity of about 25cm/sec is needed for erosion to occur False (Approx 150cm/sec would be needed)

For particles bigger than 0 For particles bigger than 0.3mm, the faster the river velocity, the larger the particle it will transport True

Clay particles are larger than silt or sand particles False

Clay particles need velocities in excess of 500cm/sec to move them False (The smallest clay particles do need 500cm/sec to move them, but larger clay needs as little as 50cm/sec. This relates to initial movement, once moving, the smallest clay particles need very little river flow to keep them moving)